Articles incorporating fluorescent dyes into polymeric matrices are extensively known in the art for various applications including signage, vehicle markings, roadway markings, and other applications where high visibility is desired and beneficial for any number of reasons, including safety, information dissemination, visibility, visual signaling, and quick detection. The extraordinarily bright appearance of fluorescent materials is what provides this enhanced visibility, which is especially pronounced at dawn and dusk. In some applications, it is important to meet and maintain certain color standards and/or certain durability standards.
Often these polymer systems containing fluorescent colorants are structured in the form of a sheeting that exhibits fluorescing properties. Particularly suitable applications for these types of films loaded with fluorescent colorants are in connection with uses where signaling is a primary function of the article. Traffic safety and informational signs have been known to incorporate films having fluorescent colorants, which enhance visibility of the signs. Certain types of signage need to have long-term outdoor durability.
It is well-known that ultraviolet radiation causes colorants, and in particular, fluorescent colorants, to degrade. When exposed to sunlight or other sources of ultraviolet (UV) light, fluorescent colorants can fade very quickly. This especially creates problems for traffic and roadway signing applications because the rapid fading of the fluorescent color can dramatically shorten the life of the sign. Past efforts to enhance the UV light durability of fluorescent articles, such as retroreflective sheeting, include the use of an UV light screening layer over or in front of a fluorescent layer. Typically, such a UV light screening layer is made by dissolving UV light absorbing compounds into a transparent polymeric matrix and then depositing the screening layer in front of a fluorescent color layer. However, approaches such as these often fail to consider and/or address potential negative interaction between the UV absorber in the screening layer and the fluorescent dye within the underlying colored layer. While UV screening is intended to address the outdoor durability problem, several difficulties can arise. One concern is that the UV light absorbing compounds of these screening layers can leach out with time or can diffuse or migrate into the underlying fluorescent layer. This diffusion can actually accelerate fading of the fluorescent colorant in certain instances. Other methods of enhancing the durability of fluorescent colorants involve using stabilizers of the hindered amine light stabilizer type (HALS type) in combination with the fluorescent colorants in a polymer matrix.
While these methods address the problem of color degradation due to UV light, they do not address the problem of color degradation caused by visible light. Often visible light is more harmful than UV light to fluorescent colorants. Where fluorescent colorants are susceptible to visible light, the use of UV stabilizers and/or UV screening layers will not be effective against color degradation due to visible light.
Other factors that may cause degradation of the fluorescent properties of a fluorescent article include the leaching out of the fluorescent colorant, the light instability of the fluorescent colorant itself and/or negative interactions among the fluorescent colorants, the polymer matrix, additives and/or impurities in the polymer.